1. Field of the Invention
The present invention relates to a method of regulating the amount of reducing agent added, especially NH.sub.3 in the case of catalytic reduction of NO.sub.x of flue gases which originate from a combustion installation which is fired with fossil fuels; the adjustment value for the quantity to be added is determined in response to a prescribed reducing agent/NO.sub.x stoichiometry factor from the quantity of combustion air supplied to the combustion installation or from the exiting quantity of flue gas, and from the NO.sub.x concentration downstream ahead of the catalyzer; the method includes a step of influencing the stoichiometry factor.
When fossil fuels are burned, there results in addition to other noxious materials, NO.sub.x which greatly pollutes the environment. In order to reduce the NO.sub.x emissions of combustion installations fired with such fuels, methods are known for effecting a reduction of the NO.sub.x to molecular nitrogen and water vapor using a reducing agent, such as NH.sub.3, in the presence of a catalyst such as vanadium compounds on a titanium oxide carrier.
2. Description of the Prior Art
The state of the art is to meter the reducing agent as a function of the flue gas quantity arising from the combustion unit, and of the NO.sub.x concentration downstream ahead of the catalyzer. As an alternative to the flue gas quantity, the quantity of combustion air supplied to the combustion installation is also frequently utilized as the reference value. The adjustment value for the addition of the reducing agent results form a multiplication operation of this input signal with a prescribed reducing agent/NO.sub.x stoichiometry factor. Depending upon the size of the installation and the construction of the reducing unit (effective catalyzer contact surface), the stoichiometry factor is set to a constant value of 0.7 to 1.0. However, this fixing requires that the maximum occurring NO.sub.x concentration is the basis for the reference value for the stoichiometry factor in order to be able to maintain the statutory emission standards.
Not taken into consideration with the heretofore known metering method is that the catalyzer is subjected to a certain amount of aging as a result of contamination due to the SO.sub.x in the flue gas. The dust particles in the flue gas have a similar effect; these particles lead to contamination of the contact surfaces of the catalyzer, and to mechanical wear of the catalyzer material due to the impact of pressure. Taken as a whole, there is therefore a reduction of the activity or displacement of the activity curve of the catalyzer as the duration of operation increases. Due to the fixed, prescribed stoichiometry factor, with the heretofore known regulating process, during the course of the operating time, an excess of the reducing agent is therefore added.
This excess of non-reacted reducing agent initially not only involves an increase of the operating costs, but also involves a series of technical problems. The reducing agent which is not reacted can enter into compounds with the noxious materials contained in the flue gas, with the dew points of these compounds being below the cold gas temperature of the air preheater which is connected after the combustion installation. Thus, for example from the ammonia added as the reducing agent, and from the SO.sub.x from the flue gas, there is formed ammonium sulfate and ammonium bisulfate, which as a result of the cooling-off of the flue gas in the air preheater below a temperature of about 220.degree. C., are deposited on the surfaces of the heat exchanger. As a result, the heat transfer is reduced, so that the service life of the heat exchanger surfaces of an air preheater are not only limited by corrosion, but also by the excessively added quantity of reducing agent. Not to be underestimated is also the fact that the marketing chances for the end product which is produced during the subsequent desulfurization of the flue gas, for example for a subsequent use as building material, becomes worse to the same extent that the concentration of impurities, i.e. also the reducing agent which has not reacted, increases. Dumping of the reaction products produced during the desulfurization of the flue gas cannot be considered as a possible alternative due to the high cost connected therewith, as well as the possible environmental pollution connected with these materials.
If these drawbacks are relevant to a combustion installation which is operated in a normal operation, these problems are multiplied for generating stations which, in order to adapt to the energy requirement at any given time, are frequently started and stopped, or are operated in low partial-load ranges. The reason for this lies in the temperature-dependent activity curve of the catalyzer. Depending upon the material, the catalyzer develops maximum activity thereof at a flue gas temperature of between 250.degree. to 400.degree. C., with a sharp drop at lower temperatures.
Especially when a combustion installation is started, but also when stopped or operating at extremely low partial load, the flue gases discharged from the installation have a low temperature. When the heretofore known regulation process is used with a fixed, set stoichiometry factor which is designed for normal load, a high excess of reducing agent which does not react is therefore produced after the catalyzer.
An object of the present invention is to further develop a method of the aforementioned general type in such a way that the amount of reducing agent added is optimized for all load situations of a combustion installation, so that an excess of reducing agent downstream after the catalyzer is avoided. At the same time, the reduction of the catalyzer activity as the operating time proceeds should be taken into consideration .